Graphene, comprising nanoparticles of carbon in single or a few layers of atoms thick, is made from graphite. These nanoparticles consist of small stacks of graphene that are 1 to 15 nanometers thick with diameters ranging from sub-micrometre to 100 micrometres. In addition to transparency and light absorption, it is the thinnest compound known to man at one atom thick, the lightest material, the strongest compound discovered, the best conductor of heat at room temperature and also the best conductor of electricity. Graphene is also flexible and capable of withstanding high stress. Therefore, it is very attractive for the application of flexible electronic devices, such as touch screens. Accordingly, there are considerable efforts to produce graphene in large quantities with good quality.
While high-quality graphene proved to be surprisingly easy to isolate in a small quantity, it is rather challenging to make a large quantity of graphene for commercial use. Common methods for preparing graphene fail into two categories: exfoliation and growth.
Exfoliation is a method of peeling (detaching) graphene layers from an existing graphite crystal. One exfoliation method is the Scotch Tape Method in which graphene is detached from a graphite crystal using an adhesive tape. Although this exfoliation method produces very high quality graphene (almost with no defects), it is labor-intensive and unsuitable for producing large amounts. The other exfoliation method is the so-called Dispersion of Graphite in which graphite crystal is dispersed in an organic solvent with nearly the same surface energy as the graphite. The energy barrier, which is required to overcome in order to separate the graphene layer, is reduced and easier for exfoliation. The solution is then subjected to an ultrasonic bath for an extended period of time. The quality of the graphene flakes obtained is very high, but size is small. However, this method enables preparing large amounts of graphene. Another method is the exfoliation of graphite oxide, which involves dispersing graphite oxide in water and then subjecting it to sonication or stirring. Graphene is then obtained by thermal or chemical methods. The performance of this method is similar to liquid-phase exfoliation, thus suitable for preparing large amounts, but quality is poor. Besides, this method employs complex equipment and involves chemicals which are not environmentally friendly.
Graphene can be grown directly from a substrate or added by chemical vapor deposition (CVD). Although these methods produce large amounts of graphene, they involve toxic chemicals and thus need complex equipment to control the process.
An improved method for preparing graphene is thus highly desirable.